1. Field of the Invention
This invention relates to a method of producing silicone elastomeric sealants based upon alkoxy functional polymers, alkoxy functional crosslinkers, and titanate catalysts that have improved shelf life in that they do not lose the ability to cure upon exposure to moisture as they are shelf aged.
2. Background Information
One of the methods of producing silicon sealants at the present time is based upon the use of alkoxy endblocked polymers and a titanium based catalyst. Such sealants are stable in the absence of moisture, but cure in the presence of moisture to a silicone elastomer. The distinguishing feature of this system over the older moisture-curing systems is the absence of acidic or corrosive byproducts produced by the curing system.
Experience with this system in commercial use has now disclosed another variable in this system in comparison to the other systems. With all other moisture-curing systems, as the sealant ages in the storage tube, it gradually cures, due to the inevitable reaction with moisture during long storage periods. This gradual cure is manifested in the gradual thickening of the sealant. When use of the over-aged sealant is attempted, the sealant cannot be expelled from the storage tube because it is too viscous or it has completely cured. In contrast to this, the alkoxy-titanate system gradually loses the ability to cure with time of storage. This is particularly undesirable because the sealant can be expelled from the tube into the desired location for sealing without the user being aware that there is anything wrong. It is only after the sealant fails to cure that a problem becomes apparent, and by then it is too late as the sealant is already in place. It is then necessary to physically remove all of the old non-curing sealant and replace it with new. This is a very time consuming and expensive process. Because of this different type of failure mode of the cure system, it became imperative that a method of producing a non-acid, non-corrosive silicone sealant be developed that would not become non-curing upon long time aging.
There are many patents directed to the system of producing silicone sealants based upon the use of alkoxy functional polymers, alkoxy functional crosslinkers, and titanate catalysts. Representative of these is U.S. Pat. No. 3,334,067, issued Aug. 1, 1967, to Weyenberg. Weyenberg discloses a method of making one component room temperature curing siloxane rubber. His compositions are stable in the absence of moisture, but cure upon exposure to moisture. The method comprises mixing in the absence of moisture a hydroxy endblocked siloxane polymer, a silane of the formula R'Si(OR")3 and a beta-dicarbonyl titanium compound.
In U.S. Pat. No. 3,383,355, issued May 14, 1968, Cooper discloses polymers having alkoxy groups bonded to terminal silicon atoms by reacting a hydroxylated organosiloxane polymer with an alkoxy silane in the presence of a suitable catalyst. He discloses that such functional diorganopolysiloxanes having from two to three alkoxy radicals attached to each terminal silicon atom are curable in the presence of moisture and a suitable hydrolysis and condensation catalyst to a rubbery material.
Smith et al., in U.S. Pat. No. 3,856,839, issued Dec. 24, 1974, disclose alkanedioxy titanium chelates which catalyze the cure of a composition which also contains methyltrimethoxysilane and a silanol chain-stopped polydiorganosiloxane fluid. The particular chelated titanium compound is stated to be desirable because it does not cause thickening during the manufacture of the composition as does the previously known titanium compounds.
An improved version of the above compositions is disclosed by Getson et al. in U.S. Pat. No. 4,111,890, issued Sept. 5, 1978, in which the hydrocarbonoxy groups linked to the organopolysiloxane, organosilicon compound and the titanium ester groups are the same. They disclose that previous compositions have a short shelflife even when kept under substantially anhydrous conditions, and that the longer these compositions are stored, the lower the property profile becomes.
It is disclosed in U.S. Pat. No. 4,438,039, issued Mar. 20, 1984, that the shelf life of some of the commercial compositions was determined by an appearance problem, manifesting itself in the formation of various sizes of crystals ranging from fine sand-like to pellet-like particles. This patent discloses a particular titanium catalyst which does not form nodules upon storage.
None of the prior art as discussed above discloses any solution for the problem of failure to cure after prolonged storage that is discussed above. After a prolonged investigation into the possible causes of such a storage failure, the solution to the problem was discovered. Once the solution to the problem is known, other background information becomes of interest.
U.S. Pat. No. 3,122,522, issued Feb. 25, 1964 to Brown and Hyde discloses a siloxane composition, each molecule of which consists essentially of (1) at least two unit of the formula EQU [R'(OCH.sub.2 CH.sub.2).sub.c O].sub.a (R.sub.b)SiZ(R.sub.2)SiO.sub.0.5
and units of the formula EQU R.sub.d SiO.sub.(4-d)/2
where each a has a value ranging from 2 to 3, each b has a value ranging from 0 to 1, the sum of a and b in any unit (1 ) is no greater than 3, each c has a value ranging from 1 to 2, each d has a value ranging from 0 to 2, and Z is a divalent hydrocarbon radical of from 2 to 18 inclusive carbon atoms, Z being free of aliphatic unsaturation. Each molecule of the silicone composition has an average of from 1.3 to 2 R groups per silicon atom and there are at least 7 units of R.sub.d SiO.sub.(4-d)/2 per molecule.
U.S. Pat. No. 3,175,993, issued Mar. 30, 1965 to Weyenberg discloses a composition consisting essentially of the average formula ##STR1## in which each R is free of aliphatic unsaturation, Z is a divalent hydrocarbon radical free of aliphatic unsaturation, y has a value of from 0 to 2 inclusive, x has a value of at least 3 and a has an average value from 2 to 3 inclusive.
Both of the above references teach preparation of the siloxane by reacting siloxanes containing --SiH groups with the appropriate silane containing a monovalent hydrocarbon radical containing an aliphatic or cycloaliphatic group in the presence of a platinum catalyst through the reaction of the --SiH and aliphatic C.dbd.C group. This reaction produces the divalent Z radical. Alternatively, the C.dbd.C group can be on the siloxane and the --SiH can be on the silane.
European Patent Application 0110251, published June 6, 1984, discloses a process for producing alkoxy-terminated polysiloxanes useful to produce room temperature vulcanizing silicone rubber compositions. The process anhydrously reacts a silanol or vinyl siloxane with a polyalkoxy crosslinking agent which is an alkoxy silane in the presence of a platinum catalyst. This alkoxy-terminated polysiloxane can also be mixed with treated filler and condensation catalyst. This application teaches that an alkoxy-terminated polysiloxane having no silethylene linkage at the polymer terminal silicon is equivalent to a polydiorganosiloxane that does contain a trialkoxysilethylene terminal group.
European Patent Application 123 935 A, published Nov. 7, 1984 by Toten and Pines, discloses an alkoxysilyl functional silicone including at least one functional group of the formula ##STR2## where w is an integer of from 2 to about 20; useful as capable of imparting satisfactory lubricity and other properties such as softness to a variety of textile fabrics.
None of the disclosures in this background information is of any assistance in solving the problem of how to improve the shelf life of silicone sealants that lose the ability to cure upon long time storage in the absence of moisture, said sealants being catalyzed with titanium compounds; because these references do not contain any comments upon the problem.